Diffusional effects for the oxidation of SiC powders in thermogravimetric analysis experiments

Abstract

The oxidation behavior of SiC powders was studied using a thermogravimetric analysis (TGA). The effects of temperature (910–1010 °C), particle size (120 nm, 1.5 μm), and the initial reactant mass on the oxidation behavior of SiC were investigated. Kinetics analysis showed that intra-particle diffusion and chemical reaction controls the oxidation rate. Moreover, a new kinetics model was proposed to describe the oxidation rate of where all diffusion steps (bulk, inter- and intra-diffusion) and the chemical reaction may affect the overall reaction rate. The model was validated through a comparison with the experimental results obtained from the oxidation of SiC powders in TGA experiments. It was found that during the experiments, inter diffusion must also be taken account to describe adequately the oxidation rate. Numerical analysis indicated that inter-particle diffusion has a significant effect on the oxidation rate, especially for larger system.

The concentration of gas reactant at the fresh SiC/oxide layer interface of particle, \( C_{{{\text{As}},}} \) is calculated according to the flux balance between inside of powder and the reaction rate \( ( - r_{\text{SiC}}^{''} = k^{''} C_{\text{As}} ) \) at the interface as follows:

By substituting \( {{\Upphi}}\left( {z,t} \right) \) obtained from Eq. 38 and using the following non-dimension parameters, the previous PDE system is changed to the one nonlinear second-order differential equation with the following boundary conditions:

At the \( t = {{\uptau}}_{\text{r}} + {{\uptau}}_{\text{s}} = \frac{1}{\beta }(1 + \frac{{6D_{\text{e}}^{s} }}{{r_{0} k^{''} }}) \) all the powders at the top surface of the bed are completely reacted. When the time is more than τ the system is divided into two parts: a part that all of the particles are completely reacted (Z* < Z < 1) and a part where the gas solid reaction is occurred.

For the part that all of the SiC particles are completely reacted (Y = 0 \( r_{\text{c}} \) = 0) then Eq. 40 is simplified to: